This invention pertains to the control of dc/dc converters, whether they are isolated or non-isolated.
Some loads, such as microprocessors and memory, draw currents that undergo fast transients. When they do, the dc/dc converter that provides power to these loads must maintain its output voltage substantially constant in order for the load to function properly. As an example, the current drawn by a microprocessor available today makes a 25 amp change in less than 1 xcexcs, during which time its dc supply voltage must not deviate from its nominal value by more than 5%.
A converter may include linear feedback to control the duty cycle of switching elements and thus maintain a desired output. The size of a dc/dc converter""s filter elements dictates the speed with which it can respond to a load current transient. These, in turn, are determined by the converter""s switching frequency and the amount of ripple that can be tolerated in the converter""s input and output waveforms. The higher the switching frequency and the larger the ripple, the smaller the filter elements can be, and the faster the converter can respond to a load transient.
Unfortunately, the higher the switching frequency the lower the converter""s efficiency. For many of today""s most demanding loads, the switching frequency required to address the transient requirements gives too low an efficiency.
One way to get around this problem is to xe2x80x9cinterleavexe2x80x9d dc/dc converters. With this well-known technique, several dc/dc converters provide the total power required. For example, two or more buck converters (comprised of xe2x80x9cmainxe2x80x9d and xe2x80x9cfreewheelingxe2x80x9d semiconductor switches, inductors, and capacitors) are placed in parallel, with each intended to carry an equal fraction of the total load current. The individual converters all switch at the same frequency, but the switch instants of each converter are phased uniformly over the switching cycle relative to the respective switch instants of the other converters. For example, two converters may be 180 degrees out of phase with respect to each other, three converters may be 120 degrees out of phase and so on. Consequently, the ripple waveforms created by each individual converter are phased with respect to the ripple waveforms of the other converters, and when they are added they cancel each other to a considerable extent.
With this cancellation of the ripple, designers can specify larger individual ripple levels, and therefore smaller filter elements, for each of the individual converters. These smaller filter elements then allow the collective converters to respond much more quickly to a load transient than could a single, more powerful converter switching at the same frequency. The technique of interleaving therefore achieves a faster response without having to raise the switching frequency and suffer the reduction in efficiency that would result.
Another well-known approach used to achieve fast response from a dc/dc converter is called xe2x80x9cbang-bangxe2x80x9d control. With this approach, the control circuit monitors the output voltage. If it falls below a threshold level set, for example, at 3% below the nominal value of the output, the control circuit immediately raises the converter""s duty ratio to its maximum value. This causes the converter""s output current to rise as fast as it can. If, on the other hand, the output voltage rises above a threshold level set, for example, at 3% above the nominal output value, the control circuit immediately lowers the converter""s duty ratio to its minimum value. This causes the converter""s output current to fall as fast as it can. When the output voltage is within the window formed by these two threshold limits, a linear feedback loop controls the duty ratio such that the output voltage settles to its nominal value when load transients are not occurring.
As a modification to this approach, some control circuits use a simpler xe2x80x9cbangxe2x80x9d control in which only one threshold level (say at 3% below nominal) is used to override a linear feedback control loop. This approach might be chosen when the converter is required to respond quickly to only one polarity of a load current transient.
The techniques of interleaving and bang-bang (or bang) control are sometimes combined to further improve a converter""s speed of response.
The control circuit that drives multiple interleaved converters must maintain a reasonable balance of power among the individual converters. One way to do this is to sense the current in each converter and to provide circuitry that modifies the duty ratio of each converter such that its current matches that of the others. The bandwidth of this current balancing circuitry is typically low (by a factor of 10 or more) compared to the bandwidth of the linear feedback loop that controls the converter""s output voltage.
With such a current balancing technique, a problem arises when bang-bang (or bang) control is used. When the bang-bang control feature is activated, the normal cyclic operation of the individual converters is disturbed in an unequal manner.
For example, assume that the output voltage falls below its lower threshold value and that the duty ratios of all the converters are set to 100% (i.e., the main switch of each converter is turned on and the freewheeling switch is turned off). During this xe2x80x9cbang interval,xe2x80x9d which lasts until the output voltage rises back above the lower threshold (perhaps with some hysteresis), some of the converters might have had their main switch on anyway, and so their operation would be unaffected. Other converters might have had their freewheeling switch on for the full bang interval, and so their operation would be significantly affected since they would xe2x80x9cmissxe2x80x9d a portion of their freewheeling interval. Of course, a converter might have had its freewheeling switch on for only a portion of the bang interval, and so its operation would be only partially affected as it misses a smaller amount of its freewheeling interval.
In all cases of this example, the xe2x80x9caffectxe2x80x9d is to raise the current level in a converter above the level it would have been had the bang control not been activated. Since the affect on each converter is unequal, the result is that the current levels in the various converters are no longer equal.
A similar result can occur if the output voltage goes too high and the bang-bang controller forces the duty ratio to 0% (i.e., all the freewheeling switches are turned on and the main switches are turned off). In this case, those converters that would have had their main switches on for at least a portion of this bang interval have their current levels lowered with respect to other converters that would not have.
If only a single disruption like this occurs at a time, the current balancing circuitry will eventually bring the current levels in the individual converters back into balance. However, if several disruptions occur quickly compared to the bandwidth of the balancing circuitry, then it is possible that the currents in some converters will rise too high. Conversely, other converters might have their currents fall so low that discontinuous operation occurs (when the freewheeling switch is a diode) or the switch current goes negative (when a synchronous diode is used).
To overcome this problem, the invention described herein xe2x80x9cmomentarily interruptsxe2x80x9d (i.e., puts on hold) the oscillator circuitry that provides the cyclic switching action of the individual converters during the bang interval. During this interruption, the control circuit turns on either the main switches or the freewheeling switches of all the converters, depending on the action required. Once the bang interval is over, the invention xe2x80x9cenablesxe2x80x9d the oscillator circuitry (i.e., the oscillator is allowed to run again, starting at the same point in its cycle where it had been interrupted). At this point, each converter goes back to having either the main switch or the freewheeling switch turned on, whichever is appropriate for its location in its switching cycle.
By interrupting the oscillator circuitry, the bang interval affects all of the individual converters equally, and none misses a portion of its normal operating cycle. The currents in each converter therefore rise (or fall) exactly the same amount (assuming all other things are equal) during the bang interval. The build-up of a current imbalance due to repeated disruptions is therefore avoided.
Thus, in accordance with the present invention, a first power converter is responsive to a first oscillator signal to provide a first power to an output. A second power converter is responsive to a second oscillator signal, out of phase with respect to the first oscillator signal, to provide a second power to the output. Override circuitry overrides the first and second oscillator signals to change the output power of both power converters to correct the output. With removal of the override, the first and second oscillator signals continue from points in their cycles at which they were overridden.
In one embodiment, the power converters are voltage converters, and the power output of each power converter is changed by changing current in an output inductor.
At least one oscillator driving the first and second oscillator signals may be frozen when the first and second oscillator signals are overridden. In the oscillator, cyclic charging and discharging of a capacitor may be interrupted during override so that the capacitor voltage remains substantially constant during the override. A single oscillator may drive both oscillator signals.